Vocab link in menu busted

[galund]

Link in menu bar doesn't work because in Github pages we're not at the root of the domain. Might as well fix this even tho it will hopefully soon be moot.

[GDS-Dominic]

Closing issue as not an issue with new theme

[galund]

In the new theme, the we've lost some mandatory navigation elements ie this needs to work like all the other bits of documentation we have - see https://www.sign-in.service.gov.uk/documentation

So I think I'd better re-open this to cover some work in that area anyway, sorry!

I think we need

• "GOV.UK One Login BETA" linking to https://www.sign-in.service.gov.uk/
• "Vocabulary" with the blue highlight linking to our root page
• bonus points if we can also link to other bits of the documentation (and then persuade them to link to us, once this is on the proper domain)

Doing the second point might resurrect the original bug unless we're careful, of course :/